The present invention relates to a semiconductor memory device and a driving method thereof, more particularly to a semiconductor memory device based on a termination operation applied to data lines and a driving method thereof.
As the amount of data for a semiconductor memory device to process increases, a chip size of the semiconductor memory device also increases. This increase in the chip size allows the elongation of data lines. For instance, for a 512 MB-level semiconductor memory device, there usually exist 4 banks, and for a 1 GB-level semiconductor memory device, the number of banks increases to 8 banks. Hence, the loading of data lines for transporting data to be accessed at a specific bank increases as much as the increased number of the bank, for instance, nearly by 2-fold.
Data lines used in semiconductor memory device are classified into segmental input/output lines, local input/output (LIO) lines, and global input/output (GIO) lines depending on the position of the data lines. In particular, since the GIO lines have a relatively heavier loading than other input/output lines, data are more likely to be distorted or damaged. Thus, a repeater operation in which two inverters are placed in an approximate mid point of the GIO lines is implemented to overcome the data distortion or damage. However, the repeater operation is merely one approach of using a driver even if the driver has an improved structure. For the repeater mode, a high amount of current is generally required due to a large number of transistors used and enlarged drivers. Accordingly, applying a termination operation to the GIO lines is another suggested approach to overcome this limitation.
According to the termination operation, prior to loading data onto the GIO lines, a voltage level of the GIO lines is pre-charged to a voltage level of a termination voltage V_TERM, for instance, ½ of a voltage level of a power supply voltage VDD. In an interval in which data are supplied, a termination operation is performed, so that a voltage level change (i.e., a width of a swing) caused by the data loaded onto the GIO lines can be reduced. As a result, the reduced swing width of a voltage in the GIO lines allows a less amount of current to be consumed, and a time parameter, usually represented with denotation ‘tAA,’ in a semiconductor memory device can be less variable.
FIG. 1 is a block diagram of a conventional semiconductor memory device. During a read operation, although not illustrated, data stored on a cell are transmitted t a read sense amplifier (SA) 20 through a LIO line LIO. The data amplified in the read SA 20 are transported to a data output mux 30 through a GIO line GIO. The data muxed in the data output mux 30 are output to a pad 50 through a pipe latch 40. During a write operation, a write SA 60 amplifies data input through the pad 50 and transports to a write driver 70 through the GIO line GIO. The write driver 70 drives the data, which are subsequently stored in the cell through the LIO line LIO.
A voltage level of the GIO line GIO fully swings from a voltage level of a power supply voltage VDD to a voltage level of a ground voltage VSS according to data. However, a termination circuit 10 performs a termination operation prior to loading data onto the GIO line GIO, so as to pre-charge a voltage level of the GIO line GIO to a voltage level that is one half of the voltage level of the power supply voltage VDD. Although the data are supplied, because of the continuously performed termination operation, the voltage level of the GIO line GIO does not fully swing but swings within a range of VDD/2 +ΔV (i.e., logic threshold voltage) and VDD/2 −ΔV.
FIG. 2 is a signal timing diagram for illustrating a width change in a voltage level of the GIO line GIO when the termination circuit 10 operates. When the termination circuit 10 does not operate, for a signal OFF_TERM that instructs the termination of the termination operation, the voltage level of the GIO line GIO fully swings from a voltage level of a power supply voltage VDD to a voltage level of a ground voltage VSS. On the other hand, when the termination circuit 10 operates, for a signal ON_TERM that instructs the performance of the termination operation, the voltage level of the GIO line GIO swings within a range between VDD/2 +ΔV and VDD/2 −ΔV.
FIG. 3 is a circuit diagram of the termination circuit 10 illustrated in FIG. 1. The termination circuit 10 includes an N-type channel metal oxide semiconductor (NMOS) transistor NM1, a P-type channel metal oxide semiconductor (PMOS) PM1, a PMOS diode PMD1, an NMOS diode NMD1, and resistors R1 and R2. The NMOS transistor NM1 turns on in response to a termination enabling signal EN_TERM, and the PMOS transistor PM1 turns on in response to an inverted signal of the termination enabling signal EN_TERM. The PMOS diode PMD1 and the NMOS diode NMD1, and the resistors R1 and R2 are coupled respectively together between the NMOS transistor NM1 and the PMOS transistor PM1. The termination circuit 10 may further include a latch block 11 latching data loaded onto a GIO line GIO.
The termination circuit 10 usually executes a termination operation when the termination enabling signal EN_TERM is in a logically high state, and stops the termination operation when the termination enabling signal EN_TERM is in a logically low state. Thus, during the execution of the termination operation, a voltage level of the GIO line GIO is precharged to a voltage level of a termination voltage V_TERM. When data are loaded onto the GIO line GIO, the voltage level of the termination voltage V_TERM conflicts with that of the data, and thus, the full swing of the voltage level of the GIO line GIO can be prevented.
The latch block 11 is activated when the termination enabling signal EN_TERM is in a logically low state, and latches a logic high or low value to a target according to the data loaded onto the GIO line GIO, so as to prevent a floating event in the GIO line GIO.
FIG. 4 is a signal timing diagram of a termination operation by the termination circuit 10 illustrated in FIG. 3. An internal read pulse (IRDP) is a signal generated in response to a read command, and an input/output strobe pulse (IOSTBP) is a signal used to enable the read SA 20 (see FIG. 1). A signal labeled as ‘YBSTC’ is a signal that becomes a logic high in response to the internal read pulse IDP, and transits from the logic high to a logic low according to the length of a burst. A termination enabling signal EN_TERM is a signal that is set to a logic high when the ‘YBSTC’ signal becomes a logic high, and reset to a logic low after a certain delay time when the ‘YBSTC’ signal transits to a logic low.
For instance, when data having a logic low are read under the state in which the GIO line GIO is latched with a logic high, the termination enabling signal EN_TERM is enabled, so that a voltage level of the GIO line GIO gradually decreases to a voltage level of the termination voltage V_TERM. When the input/output strobe pulse IOSTBP is enabled at the voltage level of the termination voltage V_TERM, the read SA 20 (se FIG. 1) operates to decrease the voltage level of the GIO line GIO. The data output mux 30 (see FIG. 1) senses the lowered voltage level of the GIO line GIO, and the GIO line GIO sustains again the voltage level of the termination voltage V_TERM.
According to the conventional circuit configuration described above, the termination circuit 10 often dissipates current due to the generation of a direct current path in an interval in which the termination enabling signal EN_TERM is enabled. In an attempt to reduce the current dissipation, a resistor that has high resistance is added to the termination circuit 10. However, it may take a long time for the GIO line GIO to reach the voltage level of the termination voltage V_TERM when the termination operation is performed.
FIG. 5 is a signal timing diagram to show potential limitations associated with the conventional termination circuit 10 illustrated in FIG. 3. Since the signals illustrated in FIG. 5 are substantially the same as those described FIG. 4, detailed description thereof will be omitted.
A resistor that has high resistance is additionally provided to the termination circuit 10 to minimize the current dissipation. Alternatively, the voltage level of the GIO line GIO may not be sufficiently reduced to the voltage level of the termination voltage V_TERM according to parameters such as process, voltage and temperature (PVT). In such a case, even though the read SA 20 operates in response to the input/output strobe pulse IOSTBP, the voltage level of the GIO line GIO may not be sufficient for the accurate detection by the data output mux 30. In particular, this insufficient voltage level of the GIO line GIO often occurs when the data latched on the GIO line GIO is different from the data that the read SA 20 is to drive. In a severe case, the data output mux 30 senses data opposite to the data that the read SA 20 is to drive. This opposite sensing of the data output mux 30 may reduce the reliability of data and circuit operations.